Identifying Tourmaline is pretty easy because it has unique identifying characteristics that rarely overlap over gems and miners. With that being said there are quite a few varieties of Tourmaline and being able to ID them quickly and accurately can be a challenge.
Learn everything there is to know about Tourmaline varieties by going here.
After you identify Tourmaline correctly you’ll probably want to know its value. Well-formed Tourmaline crystals with beautiful colors and crystal structure can easily sell for thousands of dollars.
I’ll do my best to outline the most popular varieties and give a little insight on how to quickly identify them.
How to Identify Tourmaline Through Testing
There are various ways to identify rocks, minerals, crystals, and gemstones, but we will use a method I learned while attending the Gemological Institute of America. If you’ve learned a unique way to identify gems, please share it with us.
Let’s look deeper into how to identify Tourmaline like a pro.
The visual inspection starts with what form of Tourmaline you have. The questions below are relatively easy to answer, but each type will have its own process for identifying them.
Is it a cabochon? A cabochon should have a medium to high polish with little to no pitting. It is fairly common to see Tourmaline cabochons at gem and mineral shows. The most common variety would be the cat’s eye version.
Is it faceted? Tourmaline is very commonly faceted into beautiful pieces that are mounted in jewelry. Larger stones are typically collected by enthusiasts and displayed as one-off examples.
You’ll see every color of the rainbow and stones will range in size. Some will have multicolored zones while others will show color on one axis while the other axis is dark.
Is it a specimen? Tourmaline is found in different forms. You’ll better identify these forms by looking at and inspecting this mineral over time. Here’s a list of characteristics Tourmaline displays when it’s a specimen.
- Tourmaline forms in igneous and metamorphic rocks, but most gem-quality specimens form in pegmatites.
- These gems occur as long, slender to thick prismatic and columnar crystals.
- Tourmaline is usually triangular in cross-sections and often has curved striated faces.
- Tourmaline has trigonal crystal systems that fall in the hexagonal system.
Is it tumbled? It is very common to find lower-quality material that has been tumbled. It will have a medium to high polish and be full of inclusions or cracks.
Physical Properties of Tourmaline
Let’s take a look at the physical properties of Tourmaline. Knowing what to look for will help you more easily identify what you’re looking at.
Color: Black, Blue, Green, Yellow, Red, Orange, Pink, Purple, Brown, Colorless
Clarity / Transparency: Transparent to Opaque
Fracture: Conchoidal to Uneven
The Streak Test
This is a destructive test, so you must ensure that you can damage the specimen or stone if you choose to use this method. You won’t use destructive tests once you’ve developed robust knowledge in identifying rocks and minerals.
A mineral streak test is when you scrape the stone against a harder surface to see what color remains. Tourmaline always produces a white streak.
Tumbled specimens are tested by scraping samples across a piece of ungalvanized porcelain, typically known as a streak plate.
Tourmalines are the most magnetic of all the natural transparent gemstones. This is because these gems often contain high concentrations of iron or manganese.
I don’t recommend actively testing the hardness of a stone because it’s destructive in nature and doesn’t provide a definite answer to what type of stone it is. Tourmaline has a hardness of 7 to 7.5 on the Mohs hardness scale.
Refractive Index Test
Determining the refractive index, or RI, as it’s referred to by gemologists for Tourmaline, is relatively straightforward. Still, you’ll need a specific piece of test equipment and the RI fluid to go with it. Before you place the stone on the refractometer, you want to make sure you have a flat, somewhat polished surface to take a reading.
Tourmaline’s Refractive Index: 1.616 – 1.65
Each gemstone has its own RI, so discovering a sample’s RI can help you figure out what sort of stone it actually is.
Step 1 – Place a small bead of RI fluid on the metal surface of the refractometer near the back of the crystal hemicylinder (the window on which the stone will sit).
Step 2 – Place the stone facet face down on the fluid dot and slide it toward the middle of the hemicylinder crystal using your fingers.
Step 3 – Look through the viewer lens without magnification. Continue looking until you see the outline of a bubble, then look at the bottom of this bubble. Take the reading from there, rounding the decimal to the nearest hundredth.
Occasionally, you’ll run into the issue of not having a flat surface to work with. In this instance, you’ll need to leave the top of the refractometer open and hold the rounded stone with your hand. Hopefully, you can pull a reading off of the gauge.
Consider testing the birefringence, as well. Birefringence is related to RI. While doing the birefringence test, you will turn the gemstone on the refractometer six times throughout the observation period and note the changes.
Perform a standard RI test. Instead of keeping the stone still, gradually turn it 180 degrees, making each separate turn about 30 degrees. At each 30-degree mark, take a new RI reading.
Subtract the lowest reading from the highest to find the stone’s birefringence. Round it to the nearest thousandth.
Birefringence: 0.017 to 0.035
Single or Double Refraction
Tourmaline displays a very low double refraction.
The stone must be transparent for this test to be accurate and beneficial. If the light won’t pass through the stone, there is no way to test for single or double refraction.
Check for single or double refraction. Use this test on translucent and transparent stones. You can determine whether the stone is only singly refractive (SR) or doubly refractive (DR) to help identify it. Some stones can also be classified as aggregate (AGG).
Turn on the light of a polariscope and place the stone face down on the lower glass lens (polarizer). Look through the top lens (analyzer), turning the top lens until the area around the stone looks darkest. This is your starting point.
Turn the analyzer 360 degrees and watch how the light around the stone changes.
If the stone appears dark and stays dark, it is likely an SR. If the stone starts out light and remains light, it is likely AGG. If the lightness or darkness of the stone changes, it is likely DR.
Checking The Diaphaneity
Diaphaneity refers to the mineral’s ability to transmit light. For instance, some minerals are transparent or translucent. A small amount of distortion might occur when they’re thick, but light will pass through them relatively freely.
Tourmaline is translucent to opaque.
Finding The Specific Gravity
Every stone has its unique specific gravity, which helps us identify them. Specific gravity is one of the best properties to measure when identifying mineral specimens. Most minerals have a narrow range of specific gravity, so getting an accurate measurement can go a long way toward identification.
Specific gravity is a unitless number describing how heavy a mineral is compared to equal volumes of water. For example, if a mineral is three times as dense as water, it’ll have a specific gravity of three. This is useful because while two minerals might be the same size, they’ll each have a different specific gravity.
The larger the sample, the more precise the readings tend to be. Remember that this technique can only be used for single mineral or crystal masses. It will not work for minerals embedded in host rocks.
Tourmaline’s Specific Gravity: 2.9 – 3.3
As helpful as specific gravity is for identifying minerals, amateurs are usually constrained by the need for more necessary tools for the job. However, one way to work around this is to hold the specimen and note how heavy or hefty it feels compared to what you might expect a specimen of that size to weigh.
If you want to determine the specific gravity of your stone like a pro, you’ll need to invest in a higher-end scale. The OHAUS Density Determination Kit is the one gemologists use.
Identifying Rocks and Minerals Like a Pro
Hopefully, you feel confident in your practice of identifying a piece of Tourmaline after reading and applying this guide. You’ll be using the visual part of this guide the most, and you’ll get better as you interact with more gemstones. Before you know it, you’ll be identifying stones like a gemologist.
Feel free to reach out if you encounter any issues or need clarification. I’ll do my best to assist you in the identification process.